1. Field of the Invention
The present invention generally relates to a lighting circuit for a discharge lamp, which employs a rectangular-wave based lighting system, and, more particularly, to a novel lighting circuit for a vehicular discharge lamp, which is designed to prevent lighting failure from frequently occurring in a discharge lamp immediately after the lighting of the discharge lamp or at the end of the lamp's life.
2. Description of the Related Art
Compact metal halide lamps have recently been receiving greater attention as a light source that can replace an incandescent lamp. A known lighting circuit for a vehicular metal halide lamp uses a DC power source, boosts the DC input voltage by a booster circuit, and then converts the boosted voltage into an AC voltage of a rectangular waveform by a DC/AC (Direct Current to Alternate Current) converter before applying the voltage to the metal halide lamp.
FIG. 9 shows conventional of a lighting circuit. The lighting circuit a has a battery b connected via a lighting switch c to a DC booster circuit d whose output is subsequently converted to an AC signal by a DC/AC converter e. The AC output of the DC/AC converter e is supplied via an igniter circuit f to a metal halide lamp g.
In the diagram, reference symbols "sd", "se" and "sf" schematically represent the output of the DC booster circuit d, the output of the DC/AC converter e and a voltage to be applied to the metal halide lamp g, respectively. The voltages se and sf have rectangular waveforms.
At the time the metal halide lamp g is lit by an AC voltage, a re-ignition voltage is generated when the polarity of the rectangular-wave voltage is inverted. If an insufficient voltage is supplied to the metal halide lamp g at this time, lighting failure of the lamp is likely to occur for the following reason.
When the lamp is lit by a rectangular-wave voltage, the output voltage of the DC booster circuit d becomes equal to the lamp voltage of the metal halide lamp g. If a re-ignition voltage above the saddle voltage of the lamp g is generated, sufficient power will not be supplied to the lamp g from the DC booster circuit d.
FIGS. 10A and 10B present waveform diagrams schematically showing this situation, by illustrating in an enlargement of the rising waveforms of the rectangular-wave voltage sf (i.e. the circled portion in FIG. 9). FIG. 10A shows the waveform at the normal time, and FIG. 10B shows the waveform when the lamp is lit from a cold state or when the lamp is lit near the end of its life. In the diagrams, h indicates the re-ignition voltage and i indicates the saddle voltage.
It is apparent from the diagrams that the peak value of the re-ignition voltage h rises above the saddle voltage i in the situation in FIG. 10B while the peak value of the re-ignition voltage h does not exceed the saddle voltage i in the situation in FIG. 10A.